Prior art apparatus for sampling gas in operative association with a combustion appliance, such as a furnace, boiler or water heater, have typically involved the use of a mechanical device, such as a pump, or a specially configured sampling conduit to provide a pressure difference needed to sustain a flow of gas to be sampled to a gas sensor, which is operable to measure concentration of a selected gas. A device is also needed to verify that there is sufficient gas flow to the sensor for sampling purposes. Such prior art apparatus usually focus on quantitative measurement of the gas flow and may have one or more of the following disadvantages: (1) difficulty in detecting relatively low flow rates; (2) insensistivity to flow direction; (3) flow restrictions; and (4) relatively high cost.
One application of gas sampling is to detect the presence of potentially dangerous gases, such as carbon monoxide, in the products of combustion of a furnace or in the heated air supplied by the furnace to an indoor space. Products of the combustion process are typically exhausted from the furnace through a flue after passing through a heat exchanger. An air mover, such as a blower, moves the supply air across the heat exchanger, whereby heat is transferred from the products of combustion to the air. The heated air is then supplied to an indoor space. One type of fuel used in furnaces, such as furnaces used to heat residences, is natural gas. Products of combustion of natural gas usually include water vapor, carbon dioxide and sulfur dioxide, and in the event of incomplete combustion of the natural gas, carbon monoxide. Although carbon monoxide is known to be hazardous to human health, it is usually not produced in large enough concentrations to present a danger to human beings, if the furnace is operating properly. Further, whatever carbon monoxide is produced should be confined to the inside of the heat exchanger and flue, so that under normal circumstances carbon monoxide should not be present in the supply air. However, if there is a leak in the heat exchanger, products of combustion, which may include carbon monoxide, can escape into the supply air and enter the indoor space.
Prior art techniques for preventing carbon monoxide from reaching dangerous levels in an indoor space have generally involved installation of a carbon monoxide detector either in the space or in a supply air plenum or duct downstream of the heat exchanger. A carbon monoxide detector can also be used to detect carbon monoxide in the products of combustion exhausted through the flue. The detector may generate an alarm in response to the concentration of carbon monoxide being in excess of a predetermined level, which may result in the furnace being automatically deactivated. However, carbon monoxide detectors are often unreliable and susceptible to false alarms. Also, such detectors may not be sensitive enough to accurately measure relatively low concentrations of carbon monoxide because of diffusion of the carbon monoxide gas in the supply air stream and indoor space.
In lieu of using detectors to directly measure carbon monoxide concentrations, the presence, or potential presence, of carbon monoxide in the supply air stream may be inferred if a products of combustion leak is detected in the furnace. Various procedures are known in the art for detecting products of combustion leakage from furnace components, such as heat exchangers, combustion chambers and the like. One such procedure involves introducing a non-combustible tracer gas, such as a combination of methane and nitrogen, into the heat exchanger and using a gas detector to detect the presence of any tracer gas in the supply air stream. The presence of tracer gas in the supply air stream indicates a leak in the heat exchanger through which products of combustion can escape into the supply air stream. Another procedure involves introducing a fine mist of liquid fire retardant material into the return air stream upstream of the heat exchanger and determining whether there is a change in the color of the flame in the furnace combustion chamber. A change in color from blue to orange indicates a leak in the combustion chamber wall. Other techniques involve pressurizing the heat exchanger to detect leaks. The primary disadvantages of these leak detection procedures are that they require introduction of an external gas or other fluid and the presence of a service person and special equipment.
There is, therefore, a need for an improved apparatus for sampling gas in a combustion appliance. There is also a need for an improved apparatus for detecting products of combustion leakage in a combustion appliance.